1. Field of the Invention
The present invention relates to the plasma-assisted processing of gaseous media and in particular to the reduction of the emissions of one or more of nitrogenous oxides, particulate including carbonaceous particulate, hydrocarbons including polyaromatic hydrocarbons, carbon monoxide and other regulated or unregulated combustion products from the exhausts of internal combustion engines.
2. Description of the Related Art
One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of Nitrogen (NOx). Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions. Unfortunately, in practice, it is found that combustion modification techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. A variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated when they have become saturated with particulate material.
Examples of such diesel exhaust particulate filters are to be found in European patent application EP 0 010 384; U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.
In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple physical trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. U.S. Pat. No. 4,427,418 discloses the use of ceramic coated wire or ceramic fibres.
In a broader context, the precipitation of charged, particulate matter by electrostatic forces also is known. However, in this case, precipitation normally takes place upon large planar electrodes of metal screens.
GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant. In addition to removing particulates by oxidation, especially electric discharge assisted oxidation, there is disclosed the reduction of NOx gases to nitrogen, by the use of pellets adapted to catalyse the NOx reduction.
Also, U.S. Pat. Nos. 3,983,021, 5,147,516 and 5,284,556 disclose the catalytic reduction of nitrogen oxides. However, U.S. Pat. No. 3,983,021 is solely concerned with the reduction of NO to N in a silent glow discharge, the temperature of which is kept below a value at which the oxidation of N or NO to higher oxides of nitrogen does not occur. There is no mention of any simultaneous removal of hydrocarbons.
U.S. Pat. No. 5,284,556 discloses the removal of hydrocarbons from internal combustion engine exhaust emissions. The process involved is one of dissociation in an electrical discharge of the so-called ‘silent’ type, that is to say, a discharge which occurs between two electrodes at least one of which is insulated. The device described is an open discharge chamber. Mention is made of the possible deposition of a NOx-reducing catalyst on one of the electrodes.
A conventional dielectric barrier plasma assisted gas reactor such as that disclosed in specification U.S. Pat. No. 5,284,556, consists of a plasma volume situated between two electrodes at least one of which has a dielectric barrier in the form of a thick layer of an insulating medium on its inner surface. JP-A-4027414 also discloses a dielectric barrier type of reactor in which electrodes, arranged parallel to the direction of gas flow, are positioned on opposite sides of a dielectric material through which bored holes provide gas passages.
In order to generate a plasma in such a device, the potential within the space between the electrodes must reach a critical value before the plasma will ignite. The potential which appears across the main plasma volume is dependent upon the ratio of the capacitance of the dielectric layer and that of the plasma volume because these two entities create a capacitive potential divider. The potential across the plasma volume is inversely proportioned to its capacitance, that is to say, the higher the capacitance of the plasma volume, the lower is the potential difference across it. This effect can cause a serious problem if the plasma volume is filled with a gas permeable material which has a high dielectric constant, such as pellets of barium titanate, because the potential difference across such a reactor bed may never reach the critical value for the plasma to ignite unless the supply voltage is very high, of the order of tens of kilovolts which may exceed the safe working voltage of the dielectric barrier, or other high voltage components of the power supply.